Male fiddler crabs have one enlarged claw that is used both as a weapon in combat and to attract a mate. As a vital component of the crabs' long-term survival, males are able to regrow a lost enlarged claw - but there's a catch.
While regenerated claws - grown on either the left or right side - are often the same size as the crab's original, they lack distinguishable teeth. What's worse is regrown claws tend to be much more fragile and weaker than the original, giving males a disadvantage in defending their territory or burrow.
To overcome this challenge, male fiddler crabs (Uca lactea) bluff their way through fights and adapt their fighting tactics to compensate for their weaker limb. In other words, males with weaker regrown claws choose smaller, opposite-handed opponents.
This discovery was made by wildlife researchers Daisuke Muramatsu of Kyoto University and Tsunenori Koga of Wakayama University. After videotaping 138 fights between male fiddler crabs in the Waka River estuary in Japan, researchers revealed how the sneaky crustaceans use deception to their favor.
Researchers captured, measured and marked all crabs observed in sparring matches. Of the 138 fights observed, only 42 were between males with original claws and those with regrown ones. This suggests that fiddler crabs are unable to visually discriminate one from the other, as males with regrown, or weaker, claws were not preyed on more often.
Furthermore, researchers noted they witnessed no fights including a male whose lost claw had not yet grown back to its original size, suggesting crabs are unwilling to engage in fights until their claws are completely regrown.
When fully regrown, however, regenerated crabs take no mercy: to protect their inferior claw, they chose smaller and opposite-handed opponents, as claws are generally wrestled more tightly in same-handed fights.
"Fiddler crab males adapt their fighting tactics and choice of opponent depending on whether they have lost their major claw," Muramatsu said, adding that this shows animals are able to switch tactics during their lifetime.
Males with regrown claws also tend to make a show of being aggressive to ward off opponents - but this is really just a bluff, since they surrender easily when the fight escalates to a level their handicap claw can't compete with. In fact, the videotaped fights revealed that, on average, regenerated males win only two out five such encounters.
In contrast, males with their original claws intact are more likely to choose same-handed opponents. As part of their own counter-bluff tactic, they persist in fights even when their opponents are larger. In other words, they go into a fight with the assumption that their opponent has a regenerated claw and, if this turns out to be true, the regenerated male will therefore retreat.
"The findings are in line with current theories that predict that animal signals are generally honest, but each signaling system allows for deception," co-researcher Koga added.
Their study was recently published in the journal Behavioral Ecology and Sociobiology.
